[FEATURE] Add option to set an alternative domain for preview

Make it possible to set an alternative domain for preview with
TCEMAIN.previewDomain = example.com
in PageTS. It is useful for multidomain sites.

Change-Id: If69115e877be310225c0e21685df210ca7973164
Resolves: #30889
Documentation: #52781
Releases: 6.2
Reviewed-on: https://review.typo3.org/24730
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
[FEATURE] Make frame sizes in viewpage module configurable

Available sizes in viewpage module can be configured with Page
TSConfig mod.web_view.previewFrameWidths. Defining a height
property is optional.

Change-Id: I7900959bc76fc2c586307c0fd4200f9665876dd3
Resolves: #51865
Documentation: #52783
Releases: 6.2
Reviewed-on: https://review.typo3.org/23720
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters